NETWORK WORKING GROUP S. Emery Internet-Draft Sun Microsystems Updates: 4121 (if approved) November 3, 2008 Intended status: Standards Track Expires: May 7, 2009 Kerberos Version 5 GSS-API Channel Binding Hash Agility draft-ietf-krb-wg-gss-cb-hash-agility-05.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 7, 2009. Emery Expires May 7, 2009 [Page 1] Internet-Draft Channel Binding Hash Agility November 2008 Abstract Currently, channel bindings are implemented using a MD5 hash in the Kerberos Version 5 Generic Security Services Application Programming Interface (GSS-API) mechanism [RFC4121]. This document updates RFC4121 to allow channel bindings using algorithms negotiated based on Kerberos crypto framework as defined in RFC3961. In addition, because this update makes use of the last extensible field in the Kerberos client-server exchange message, extensions are defined to allow future protocol extensions. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 3. Channel binding hash agility . . . . . . . . . . . . . . . . . 5 4. Security considerations . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 7. Normative References . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12 Emery Expires May 7, 2009 [Page 2] Internet-Draft Channel Binding Hash Agility November 2008 1. Introduction With the recently discovered weaknesses in the MD5 hash algorithm there is a need to use stronger hash algorithms. Kerberos Version 5 Generic Security Services Application Programming Interface (GSS-API) mechanism [RFC4121] uses MD5 to calculate channel binding verifiers. This document specifies an update to the mechanism that allows it to create channel binding information based on negotiated algorithms. This will allow deploying new algorithms incrementally without break interoperability with older implementations, when new attacks arise in the future. Emery Expires May 7, 2009 [Page 3] Internet-Draft Channel Binding Hash Agility November 2008 2. Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The term "little endian order" is used for brevity to refer to the least-significant-octet-first encoding, while the term "big endian order" is for the most-significant-octet-first encoding. Emery Expires May 7, 2009 [Page 4] Internet-Draft Channel Binding Hash Agility November 2008 3. Channel binding hash agility When generating a channel binding verifier, Bnd, a hash is computed from the channel binding fields. Initiators MUST populate the Bnd field in order to maintain interoperability with existing acceptors. In addition, initiators MUST populate the extension field, Exts. All fields before "Exts" do not change from what is described in [RFC4121], they are listed for convenience. The 0x8003 GSS checksum MUST have the following structure: Octet Name Description ----------------------------------------------------------------- 0..3 Lgth Number of octets in Bnd field; Represented in little-endian order; Currently contains hex value 10 00 00 00 (16). 4..19 Bnd Channel binding information, as described in section 4.1.1.2 [RFC4121]. 20..23 Flags Four-octet context-establishment flags in little-endian order as described in section 4.1.1.1 [RFC4121]. 24..25 DlgOpt The delegation option identifier (=1) in little-endian order [optional]. This field and the next two fields are present if and only if GSS_C_DELEG_FLAG is set as described in section 4.1.1.1 [RFC4121]. 26..27 Dlgth The length of the Deleg field in little-endian order [optional]. 28..(n-1) Deleg KRB_CRED message (n = Dlgth + 28) [optional]. n..last Exts Extensions where Exts is the concatenation of zero, one or more individual extensions, each of which consists of, in order: type -- big endian order unsigned integer, 32-bits, which contains the type of extension length -- big endian order unsigned integer, 32-bits, which contains the length, in octets, of the extension data encoded as an array of octets immediately following this field data -- octet string of extension information in that order If multiple extensions are present then there MUST be at most one instance of a given extension type. When channel binding is used the Exts MUST include the following extension: Emery Expires May 7, 2009 [Page 5] Internet-Draft Channel Binding Hash Agility November 2008 data-type 0x00000000 data-value The output obtained by applying the Kerberos V get_mic() operation [RFC3961], using the sub-session key from the authenticator and key usage number 43, to the channel binding data as described in [RFC4121], section 4.1.1.2 (using get_mic instead of MD5). Initiators that are unwilling to use a MD5 hash of the channel bindings MUST set the Bnd field to sixteen octets of hex value FF. Emery Expires May 7, 2009 [Page 6] Internet-Draft Channel Binding Hash Agility November 2008 4. Security considerations Initiators do not know if the acceptor had ignored channel bindings or whether it validated the MD5 hash of the channel bindings [RFC4121]. Ultimately, it is up to the application whether to use channel binding or not. This is dependent upon the security policy of these applications. Emery Expires May 7, 2009 [Page 7] Internet-Draft Channel Binding Hash Agility November 2008 5. IANA Considerations The IANA is hereby requested to create a new registry of "Kerberos V GSS-API mechanism extension types" with four-field entries (type number, type name, description, and normative reference) and, initially, a single registration: 0x00000000, "Channel Binding MIC," "Extension for the verifier of the channel bindings," . Using the guidelines for allocation as described in [RFC5226], type number assignments are as follows: 0x00000000 - 0x000003FF IETF Consensus 0x00000400 - 0xFFFFF3FF Specification Required 0xFFFFF400 - 0xFFFFFFFF Private Use Emery Expires May 7, 2009 [Page 8] Internet-Draft Channel Binding Hash Agility November 2008 6. Acknowledgements Larry Zhu helped in the review of this document overall and provided the suggestions of typed-data. Nicolas Williams and Sam Hartman suggested that the Bnd and Exts fields be populated simultaneously. Nicolas Williams and Jeffrey Hutzelman had also suggested a number changes to this document. Emery Expires May 7, 2009 [Page 9] Internet-Draft Channel Binding Hash Agility November 2008 7. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2", RFC 4121, July 2005. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. Emery Expires May 7, 2009 [Page 10] Internet-Draft Channel Binding Hash Agility November 2008 Author's Address Shawn Emery Sun Microsystems 500 Eldorado Blvd M/S UBRM05-171 Broomfield, CO 80021 US Email: shawn.emery@sun.com Emery Expires May 7, 2009 [Page 11] Internet-Draft Channel Binding Hash Agility November 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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